The present invention relates to a method of controlling charge and discharge of a secondary battery for automatic guided vehicle.
An automatic guided vehicle is used for automation and laborsaving of the delivery of materials in factories and warehouses. The automatic guided vehicle mounts a secondary battery (battery) as a power source for a traction electric motor and a driving source for other loads thereon and is controlled so as to run automatically on various driving paths along a guide line such as a guide tape adhered to the floor of factories, warehouses, etc. When the battery needs to be charged or there is no request to deliver materials from an operational control station, the automatic guided vehicle moves to a charge station as a destination and is connected to a battery charger at the charge station to charge the secondary battery.
Conventionally, a lead storage battery has been used as the secondary battery mounted on such automatic guided vehicle. However, the lead storage battery takes a long time to be charged and requires equipment for its charge and reloading as well as personnel for maintenance. Accordingly, in these days, an alkali storage battery such as nickel metal hydride battery, which is capable of being quickly charged at the charge station in a short time and requires no maintenance, has become widespread in place of lead storage battery.
Generally, an automatic guided vehicle system placed in factories comprises a minimum number of automatic guided vehicles and battery chargers essential to perform desired operations so as to reduce its total cost and prevent unnecessary space from being occupied. After the secondary battery mounted on the automatic guided vehicle supplies electric power to the loads for a few hours (the automatic guided vehicle runs automatically on the driving path), the automatic guided vehicle moves to the battery charger automatically according to an instruction of a central control unit. The battery charger charges the secondary battery quickly in a few minutes. If the nickel metal hydride battery is charged fully by quick charge, a dangerous increase in its internal pressure causes and therefore the battery is not charged fully at quick charge. As a result, incomplete charge and discharge of the secondary battery is repeated. When the nickel metal hydride battery or nickel cadmium battery is incompletely charged and discharged repeatedly without full charge, memory effect of increasing a voltage drop during discharge generates. In the case where the secondary battery with the memory effect is used for equipment in which discharge end voltage is set at a fixed value, the apparent capacity decreases with drop in voltage. For the secondary battery consisting of a plurality of single cells, voltages of respective single cells differ in proportion to accumulated discharged amount, and drop in apparent capacity and terminal voltage of the secondary battery occurs.
The above-mentioned drop in apparent capacity and terminal voltage can be resolved by discharging the secondary battery fully and then performing full charge or overcharge of the secondary battery, that is, refresh charge and discharge. A battery charge control device for automatic guided vehicle disclosed in a patent document 1 (Patent Publication No. 2560878) stores the number of battery charge times and an accumulated value of charged capacity and calculates an integrated discharged amount. When the integrated discharged amount reaches a preset amount, the device requests the user to perform equalizing charge.
A method of charging and discharging a secondary battery for automatic guided vehicle of a patent document 2 (Unexamined Patent Publication No. 2002-191103) integrates the discharged amount of the secondary battery at the side of the automatic guided vehicle, and when the integrated discharged amount reaches a preset amount, requests a battery control system to perform maintenance charge and discharge (refresh charge and discharge).
The conventional methods only measure the number of incomplete charge (or discharge) times or total discharged amount and determine the timing of refresh charge and discharge. Accordingly, even if memory effect or nonuniformity of voltages of respective single cells does not occur in fact, the methods may request refresh charge and discharge, thereby to cause congestion at the charge station and then interfere with the operation in factories equipped with the automatic guided vehicle. Moreover, there is a problem that memory effect or nonuniformity of voltages of respectiive single cells may not be able to be detected, resulting in sudden stop of the automatic guided vehicle.
To solve the above-mentioned conventional problems, the present invention provides a method of controlling charge and discharge of the secondary battery for automatic guided vehicle that can decide the timing of refresh charge and discharge of the secondary battery for automatic guided vehicle accurately and minimize the frequency of refresh charge and discharge.